基于第一性原理计算的层状钠离子固态电解 质结构设计及离子输运特性研究

  全固态钠离子电池具有高安全、高比能和低成本等优势，有望应用于大 规模储能领域。作为全固态钠离子电池的关键组成，钠离子固态电解质需要 满足高的离子电导率、低的电子电导率、良好的化学稳定性、宽的电化学窗 口、良好的与电极界面相容性、低成本、易规模化制备等要求。然而，目前 对钠离子固态电解质的研究仍不多，且未有一种电解质可以完全满足以上要 求。因此，开发高性能的固态电解质是推动固态钠离子电池发展的关键。层 状氧化物固态电解质具有较高的化学稳定性和良好的离子传输性能 而备受 期待。固态电解质的电化学性能往往决定于元素组分和具体的结构特征，因 此，进一步理解结构对其性能的影响机制，对开发高性能固态电解质具有重 要意义。本文基于第一性原理计算方法，对层状钠离子固态电解质的晶体结 构、电子结构和离子输运等进行了研究，并阐释了离子掺杂和元素替换对材 料物性的影响规律。具体内容如下: 

  基于第一性原理研究层状氧化物电解质 Na₂Zn₂TeO₆（NZTO）的基本性 质、离子迁移机制，并通过异价离子掺杂调控结构以提高离子传输性 能。 NZTO 是一种钠/空位无序的层状结构，通过 Supercell 程序筛选并进行结构 弛豫，得到了能量最低的超胞结构。随后，结合第一性原理 分 子动力学 （AIMD）、弹性能带法（NEB）和键价位能计算（BVSE）对 NZTO 中 Na⁺ 的输运动力学进行了计算讨论。结果表明，NZTO 的二维层空间提供了快速 的 Na⁺迁移通道，且 Na⁺的扩散呈现出了高度的相关性。此外，通过异价离 子掺杂引入更多空位，进一步提高了 Na⁺ 的可迁移性。

  然而，较高的合成温度以及高界面阻抗通常是氧化物电解质应用面临的 困境；硫化物电解质的合成温度和界面阻抗较低，但稳定性较差。为了结合 层状氧化物的稳定性和硫化物固态电解质的低界面阻抗特性 ， 本 文 基 于 CALYPSO 程序预测了一类新型层状氧硫化物 NaMChO（M=Al，Ga；Ch=S， Se）的晶体结构。随后，结合 AIMD、声子谱、电子态密度 BVSE 等计算方 法，系统地阐明了它们的热力学、晶格动力学、电子结构和离子输运特性， 并通过声子投影态密度和 NEB 计算，探究了其结构对 Na⁺跃迁的影响机制。 结果表明 NaMChO 具有低至 30 meV 的 Na⁺迁移势垒和宽至 5 eV 的电子带 隙，是一类极具潜力的钠离子固态电解质。其中，间隙 Na⁺为主要的载流子，并通过“Kick-off”机制快速协同输运。此外，我们通过对比 NaMChO 不同 组分的结构特征和钠离子输运特性，提出了“晶格软化”和“诱导效应”对 Na⁺输运的协同调控机制。 

    All-solid-state sodium-ion batteries (ASSSIBs) are expected for large-scale energy storage due to their advantages of high safety, high specific energy and low cost. As a key component of ASSSIBs, sodium-ion solid electrolytes are required to have high ionic conductivity, low electronic conductivity, good chemical stability, wide electrochemical window, good compatibility with electrodes, low cost and easy large-scale preparation. However, there are still few studies on sodium-ion solid-state electrolytes, and it is difficult to find electrolytes that can fully meet these requirements. The exploitation of high - performance solid-state electrolytes is the key to promoting the development of ASSSIBs. Among various solid-state electrolytes, layered oxide solid electrolytes are highly anticipated because of their high chemical stability, wide transport channel for sodium-ions and high ionic conductivity. The electrochemical properties of solid-state electrolytes are often closely related to their structural characteristics. Therefore, a further understanding of the structure-performance relationship is of great significance for the exploitation of high-performance solid electrolytes. In this thesis, the crystal structure, electronic structure and ion transport of layered sodium-ion solid electrolytes are studied by first-principles calculations, and the mechanism of the influence of heteroatom doping on the electrochemical properties was clarified. The specific content is as follows:

    Based on the first-principles calculations, the basic properties and ion-transport mechanism of layered oxide electrolyte Na₂Zn₂TeO₆ (NZTO) were studied. The ion-transport performance was improved by adjusting the structure through heterovalent ion doping. NZTO is a sodium/vacancy-disordered layered structure, which is screened by the Supercell program and undergoes structural relaxation to obtain the super cellular structure with the lowest energy. Subsequently, the kinetics of Na⁺ transport in NZTO was extensively discussed using first-principles molecular dynamics (AIMD), elastic energy band (NEB) and bond valence site energy calculation methods. The results show that the two - dimensional interlayer space of NZTO provides a fast migration channel for Na⁺ , and the diffusion of Na⁺ is highly correlated. In addition, the Na⁺ mobility can be further enhanced by introducing more vacancies from the heterovalent ion doping.          High synthesis temperature and high interfacial resistance are continuing challenges to realize the application of oxide electrolytes. Sulfide electrolytes have low synthesis temperature and low interfacial resistance, but their application is limited by poor chemical and electrochemical stabilities. Combined with the stability of layered oxides and the high ionic conductivity of sulfide solid electrolytes, a reasonable structure prediction of the layered sulfur oxide NaMChO (M = Al, Ga; Ch = S, Se) was performed by the CALYPSO program. Subsequently, their thermodynamics, lattice dynamics, electronic structure and ion-transport properties were theoretically and systematically elucidated through calculations such as AIMD, phonon spectroscopy, electronic density of states and bond-valence force field methods. Through the phonon projected density of states and NEB calculations, the effect of the crystal structure on the Na⁺ transition is explored. The results show that NaMChO has a Na⁺ migration barrier as low as 30 meV and a wide bandgap close to 5 eV, which shows a high potential as solid - state electrolyte for Na⁺ transport. Interstitial Na⁺ is the main carrier and is rapidly co-transported through the "kick-off" mechanism. In addition, by comparing the structural features and sodium-ion transport properties of different components, it was found that the ion transport of NaMChO is affected by both "lattice softening" and "inductive effect".

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